The Stovebolt Site Forums The Shop Area Engine Shop 4 jet or Q jet?

im about to rebuild a rochester 4jet that came on my 66 c-30. i was just wondering what the diffrences are from a q-jet? obviously the secondary venturies are smaller, but is there any. . .performance advantage or disadvantage? would i be better putting in a q-jet? what are the average cfm on the 4jet for small blocks? (still not really sure what motor it is; 283 or 327) ive worked a lot on the q-jets, but this is the first 4 jet i have ever even seen. would it be more worth it to rebuild this 4 jet, or go to the bone yard and grab a q jet to rebuild? and is it any diffrent switching the choke to an electric choke? i think the 4 jet was a hot air choke, but the line that goes into it was cut off when i bought it, and i haven't the slightest where it used to go! and do the elect. chokes off the q-jets fit on the 4 jets? any insight is appreciated!

I cant answer all your questions but can add a little. The 4gc wasnt a bad carb one of their bad points was the gas didnt circulate thru the secondary bowls. They filled up and if you didnt use the 4 bbl the same gas set there forever. Ive seen some varnished messes on some of them. The heat tube from the choke went to a "stove" in the exhaust manifold.

The Rochester 4-Jet was produced from about 1952 through 1966. Like most other products, they did have "teething" problems. The initial design (1952~1955) had several issues. Beginning approximately 1956 (there was some overlap for different companys) the 4-GC was an excellent carburetor.

Rochester 4-Jets were produced in sizes from approximately 400 CFM to approximately 700 CFM. There is no "average" size.

There would be no significant performance increase in using a Q-Jet on smaller engines (300 CID or less). The real performance advantage with the Q-Jet is on larger engines because of the small primaries, which allows higher air velocity at low RPMS.

The 4-Jet is a more reliable design than the Q-Jet.

DON'T switch to an electric choke if you have an automatic transmission UNLESS you live in in Miami, San Diego, or Honolulu. Fix the hot air choke.

If you should decide to go with the Q-Jet, it will REQUIRE an intake manifold change. The "spread-to-square" adapters are for the seedy used car lots that switch locations after each car sale!

Either, if properly applied and properly rebuilt, are great carbs.

Jon.

i was planning on putting new manifold, but i noticed that the base of this 4jet is more narrow than the square bore holder that i have. no its not an automatic, its 4 on the floor. i just prefer the electric chokes. so (just guessing) a 283 would have about 600cfm, and a 327 would be 700 cfm (aprox)? i just cant remember the formula for figuring cfm.

Here are a couple of articles on CFM:

http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

http://www.thecarburetorshop.com/Carburetorsizeselection.htm

For a mild street engines with square bore carb, 400 CFM for a 283, and 500~600 for a 327.

With the spread-bore design, larger carbs could be used due to the small primaries. The early Q-Jets were about 750 CFM.

Jon.

most Q Jets were 750 cfm. they only flowed what the engine was asking for

hey thanks for the links to those articles carb king. good info! i finally found out that the motor is a 327. thanks- im going to have to start working out some numbers!